University of Massachusetts Amherst ScholarWorks@UMass Amherst Astronomy Department Faculty Publication Series Astronomy 2010 DUST EMISSION AND STAR FORMATION IN STEPHAN'S QUINTET G Natale RJ Tuffs CK Xu CC Popescu J Fischera See next page for additional authors Follow this and additional works at: https://scholarworks.umass.edu/astro_faculty_pubs Part of the Astrophysics and Astronomy Commons Recommended Citation Natale, G; Tuffs, RJ; Xu, CK; Popescu, CC; Fischera, J; Lisenfeld, U; Lu, N; Appleton, P; Dopita, M; Duc, P-A; Reach, W; Sulentic, J; and Yun, Min, "DUST EMISSION AND STAR FORMATION IN STEPHAN'S QUINTET" (2010). The Astrophysical Journal. 1141. 10.1088/0004-637X/725/1/955 This Article is brought to you for free and open access by the Astronomy at ScholarWorks@UMass Amherst. It has been accepted for inclusion in Astronomy Department Faculty Publication Series by an authorized administrator of ScholarWorks@UMass Amherst. For more information, please contact [email protected]. Authors G Natale, RJ Tuffs, CK Xu, CC Popescu, J Fischera, U Lisenfeld, N Lu, P Appleton, M Dopita, P-A Duc, W Reach, J Sulentic, and Min Yun This article is available at ScholarWorks@UMass Amherst: https://scholarworks.umass.edu/astro_faculty_pubs/1141 Preprint typeset using LATEX style emulateapj v. 11/12/01 DUST EMISSION AND STAR FORMATION IN STEPHAN’S QUINTET G. Natale1, R. J. Tuffs1, C. K. Xu2, C. Popescu3, J. Fischera4, U. Lisenfeld5, N. Lu2,P. Appleton6, M. Dopita7, P.-A. Duc8, Y. Gao9, W. Reach10, J. Sulentic11, M. Yun12 ABSTRACT We analyse a comprehensive set of MIR/FIR observations of Stephan’s Quintet (SQ), taken with the 43 Spitzer Space Observatory. Our study reveals the presence of a luminous (LIR ≈ 4.6 × 10 erg/s) and extended component of infrared dust emission, not connected with the main bodies of the galaxies, but roughly coincident with the X-ray halo of the group. We fitted the inferred dust emission spectral energy distribution of this extended source and the other main infrared emission components of SQ, including the intergalactic shock, to elucidate the mechanisms powering the dust and PAH emission, taking into account collisional heating by the plasma and heating through UV and optical photons. Combining the inferred direct and dust-processed UV emission to estimate the star formation rate (SFR) for each source we obtain a total SFR for SQ of 7.5 M⊙/yr, similar to that expected for non- interacting galaxies with stellar mass comparable to the SQ galaxies. Although star formation in SQ is mainly occurring at, or external to the periphery of the galaxies, the relation of SFR per unit physical area to gas column density for the brightest sources is similar to that seen for star-formation regions in galactic disks. We also show that available sources of dust in the group halo can provide enough dust to 42 produce up to LIR ≈ 10 erg/s powered by collisional heating. Though a minority of the total infrared emission (which we infer to trace distributed star-formation), this is several times higher than the X-ray luminosity of the halo, so could indicate an important cooling mechanism for the hot IGM and account for the overall correspondence between FIR and X-ray emission. We investigate two potential modes of star-formation in SQ consistent with the data, fuelled either by gas from a virialised hot intergalactic medium continuously accreting onto the group, whose cooling is enhanced by grains injected from an in-situ population of intermediate mass stars, or by interstellar gas stripped from the galaxies. The former mode offers a natural explanation for the observed baryon deficiency in the IGM of SQ as well as for the steep LX–TX relation of groups such as SQ with lower velocity dispersions. Subject headings: galaxies: groups: individual (Stephan’s Quintet) – galaxies: evolution – interactions – ISM – intergalactic medium 1. introduction the intergalactic medium (IGM) is being converted into stars. Whereas at early epochs gas fuelling of galaxies Physical processes occurring in the environments of is thought to proceed via cold gas accretion in low mass groups of galaxies play a fundamental role in determin- ing the star formation history of the Universe. Galaxy DMHs, this mechanism is predicted to be inhibited by the higher virial temperature of the IGM in high mass DMHs groups are associated with intermediate mass dark mat- ter haloes (DMH) which occupy a pivotal position in the hosting galaxy groups (Dekel et al. 2006). Furthermore, formation of structures, acting as a centre of aggrega- because of the high densities, galaxy-galaxy and galaxy- IGM interactions should become effective in removing in- tion of lower mass DMHs and their associated galaxies while being the building blocks for the most massive clus- terstellar gas from galaxies in groups, leading ultimately to ters of galaxies that form at later epochs (Springel et al. a quenching of star formation in the already existing galax- arXiv:1010.1227v1 [astro-ph.CO] 6 Oct 2010 ies which have fallen into the groups. On the other hand, 2005). In the present universe about 50 percent of all stel- lar mass is contained within groups of total mass higher these same interactions lead to a chemical enrichment as than log M/M & 12.5 (Eke et al. 2005). The group en- the IGM becomes mixed with the stripped galaxian ISMs 10 ⊙ which, potentially, could enhance the cooling of the IGM vironment affects the modality in which baryonic gas in 1 Max Planck Institute f¨ur Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany; [email protected], richard.tuffs@mpi- hd.mpg.de 2 Infrared Processing and Analysis Center, California Institute of Technology 100-22, Pasadena, CA 91125, USA 3 University of Central Lancashire, Preston, PR1 2HE, UK 4 Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 Saint George Street, Toronto, ON, M5S 3H8, Canada. 5 Department de F´ısica Te´orica y del Cosmos, Universidad de Granada, Granada, Spain 6 NASA Herschel Science Center, IPAC, California Institute of Technology, Pasadena, CA 91125, USA 7 Research School of Astronomy & Astrophysics, The Australian National University, Cotter Road,Weston Creek, ACT 2611, Australia 8 Laboratoire AIM, CEA/DSM-CNRS-Universit´eParis Diderot, Dapnia/Service dAstrophysique, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France 9 Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing 210008, China 10 Spitzer Science Center, IPAC, California Institute of Technology, Pasadena, CA 91125, USA 11 Instituto de Astrof´ısica de Andaluc´ıa, CSIC, Apdo. 3004, 18080, Granada, Spain 12 Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA 1 2 Natale et al. and its ability to accrete onto existing galaxies and to form 2008) is also shown in the obscured component of star for- new star forming systems. The relative importance of all mation, and how the pattern of total star formation in the these processes in determining the star formation activity IGM is related to the morphological distribution of gas in which is observed to be taking place in groups is an open the different temperature ranges. question. A further motivation for studying SQ in the in- The Stephan’s Quintet compact group of galaxies (SQ) frared are theoretical studies by Dwek et al. (1990) and 13 presents a natural laboratory with which all these phe- Montier et al. (2004) which have predicted that even small nomena affecting star formation in groups can be stud- amounts of dust in the hot virialised IGM could pro- ied. As shown by Trinchieri et al. (2005) (T05) and vide an important cooling mechanism via inelastic gas- O’Sullivan et al. (2009) (OS9), this group presents a dif- grain collisions, with the radiation appearing in the FIR. fuse halo of X-ray emission extending in radius at least as This has prompted searches for a FIR counterpart to the far as 40 kpc with the bulk of the gas radiating at temper- X-ray emitting intracluster medium in several rich clus- atures ≈ 6×106K. Assuming hydrostatic equilibrium, this ters (Stickel et al. 1998, Stickel et al. 2002, Bai et al. 2007, 12 indicates a dark matter halo mass of ≈ 10 M⊙, interme- Kitayama et al. 2009) which however have thus far yielded diate between galaxies and clusters. The metal abundance no unambiguous detection. This may be attributed to the of this hot gas is rather poorly constrained, consistent with very low abundance of grains predicted on the basis of a primordial and/or galaxian origin. realistic estimates of sources and sinks of grains in the One galaxy, NGC 7318b, is apparently unbound, enter- IGM (Popescu et al. 2000), the expected similarity of the ing the group at a high relative velocity of ≈ 1000 km/s FIR colors of the collisionally heated dust emission compo- and colliding with the group IGM, as evidenced by nent with photon-heated diffuse dust in foreground cirrus a ∼ 32 kpc north-south long ridge prominent in ra- (Popescu et al. 2000), and the problem of spatial confu- dio continuum (Xu et al. 2003), optical line emission sion with star forming galaxies in the cluster (Quillen et al. (Xu et al. 1999), X-ray (Trinchieri et al. 2005) and re- 1999). SQ is a good object to search for this phenomena cently also powerful mid-infrared rotational hydrogen lines since its angular size is small enough for accurate pho- (Appleton et al. 2006; Cluver et al. 2010). The galax- tometric measurements of extended emission and, at the ies of the group present extended tidal tails that have same time, it is possible to separate emission from discrete been used to constrain their recent interaction history star-forming sources, such as the constituent galaxies and (Moles et al.